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+Rails nested model forms
+========================
+
+Creating a form for a model _and_ its associations can become quite tedious. Therefore Rails provides helpers to assist in dealing with the complexities of generating these forms _and_ the required CRUD operations to create, update, and destroy associations.
+
+After reading this guide, you will know:
+
+* do stuff.
+
+--------------------------------------------------------------------------------
+
+NOTE: This guide assumes the user knows how to use the [Rails form helpers](form_helpers.html) in general. Also, it's **not** an API reference. For a complete reference please visit [the Rails API documentation](http://api.rubyonrails.org/).
+
+
+Model setup
+-----------
+
+To be able to use the nested model functionality in your forms, the model will need to support some basic operations.
+
+First of all, it needs to define a writer method for the attribute that corresponds to the association you are building a nested model form for. The `fields_for` form helper will look for this method to decide whether or not a nested model form should be built.
+
+If the associated object is an array, a form builder will be yielded for each object, else only a single form builder will be yielded.
+
+Consider a Person model with an associated Address. When asked to yield a nested FormBuilder for the `:address` attribute, the `fields_for` form helper will look for a method on the Person instance named `address_attributes=`.
+
+### ActiveRecord::Base model
+
+For an ActiveRecord::Base model and association this writer method is commonly defined with the `accepts_nested_attributes_for` class method:
+
+#### has_one
+
+```ruby
+class Person < ActiveRecord::Base
+  has_one :address
+  accepts_nested_attributes_for :address
+end
+```
+
+#### belongs_to
+
+```ruby
+class Person < ActiveRecord::Base
+  belongs_to :firm
+  accepts_nested_attributes_for :firm
+end
+```
+
+#### has_many / has_and_belongs_to_many
+
+```ruby
+class Person < ActiveRecord::Base
+  has_many :projects
+  accepts_nested_attributes_for :projects
+end
+```
+
+### Custom model
+
+As you might have inflected from this explanation, you _don't_ necessarily need an ActiveRecord::Base model to use this functionality. The following examples are sufficient to enable the nested model form behavior:
+
+#### Single associated object
+
+```ruby
+class Person
+  def address
+    Address.new
+  end
+
+  def address_attributes=(attributes)
+    # ...
+  end
+end
+```
+
+#### Association collection
+
+```ruby
+class Person
+  def projects
+    [Project.new, Project.new]
+  end
+
+  def projects_attributes=(attributes)
+    # ...
+  end
+end
+```
+
+NOTE: See (TODO) in the advanced section for more information on how to deal with the CRUD operations in your custom model.
+
+Views
+-----
+
+### Controller code
+
+A nested model form will _only_ be built if the associated object(s) exist. This means that for a new model instance you would probably want to build the associated object(s) first.
+
+Consider the following typical RESTful controller which will prepare a new Person instance and its `address` and `projects` associations before rendering the `new` template:
+
+```ruby
+class PeopleController < ApplicationController
+  def new
+    @person = Person.new
+    @person.built_address
+    2.times { @person.projects.build }
+  end
+
+  def create
+    @person = Person.new(params[:person])
+    if @person.save
+      # ...
+    end
+  end
+end
+```
+
+NOTE: Obviously the instantiation of the associated object(s) can become tedious and not DRY, so you might want to move that into the model itself. ActiveRecord::Base provides an `after_initialize` callback which is a good way to refactor this.
+
+### Form code
+
+Now that you have a model instance, with the appropriate methods and associated object(s), you can start building the nested model form.
+
+#### Standard form
+
+Start out with a regular RESTful form:
+
+```erb
+<%= form_for @person do |f| %>
+  <%= f.text_field :name %>
+<% end %>
+```
+
+This will generate the following html:
+
+```html
+<form action="/people" class="new_person" id="new_person" method="post">
+  <input id="person_name" name="person[name]" type="text" />
+</form>
+```
+
+#### Nested form for a single associated object
+
+Now add a nested form for the `address` association:
+
+```erb
+<%= form_for @person do |f| %>
+  <%= f.text_field :name %>
+
+  <%= f.fields_for :address do |af| %>
+    <%= af.text_field :street %>
+  <% end %>
+<% end %>
+```
+
+This generates:
+
+```html
+<form action="/people" class="new_person" id="new_person" method="post">
+  <input id="person_name" name="person[name]" type="text" />
+
+  <input id="person_address_attributes_street" name="person[address_attributes][street]" type="text" />
+</form>
+```
+
+Notice that `fields_for` recognized the `address` as an association for which a nested model form should be built by the way it has namespaced the `name` attribute.
+
+When this form is posted the Rails parameter parser will construct a hash like the following:
+
+```ruby
+{
+  "person" => {
+    "name" => "Eloy Duran",
+    "address_attributes" => {
+      "street" => "Nieuwe Prinsengracht"
+    }
+  }
+}
+```
+
+That's it. The controller will simply pass this hash on to the model from the `create` action. The model will then handle building the `address` association for you and automatically save it when the parent (`person`) is saved.
+
+#### Nested form for a collection of associated objects
+
+The form code for an association collection is pretty similar to that of a single associated object:
+
+```erb
+<%= form_for @person do |f| %>
+  <%= f.text_field :name %>
+
+  <%= f.fields_for :projects do |pf| %>
+    <%= pf.text_field :name %>
+  <% end %>
+<% end %>
+```
+
+Which generates:
+
+```html
+<form action="/people" class="new_person" id="new_person" method="post">
+  <input id="person_name" name="person[name]" type="text" />
+
+  <input id="person_projects_attributes_0_name" name="person[projects_attributes][0][name]" type="text" />
+  <input id="person_projects_attributes_1_name" name="person[projects_attributes][1][name]" type="text" />
+</form>
+```
+
+As you can see it has generated 2 `project name` inputs, one for each new `project` that was built in the controller's `new` action. Only this time the `name` attribute of the input contains a digit as an extra namespace. This will be parsed by the Rails parameter parser as:
+
+```ruby
+{
+  "person" => {
+    "name" => "Eloy Duran",
+    "projects_attributes" => {
+      "0" => { "name" => "Project 1" },
+      "1" => { "name" => "Project 2" }
+    }
+  }
+}
+```
+
+You can basically see the `projects_attributes` hash as an array of attribute hashes, one for each model instance.
+
+NOTE: The reason that `fields_for` constructed a hash instead of an array is that it won't work for any form nested deeper than one level deep.
+
+TIP: You _can_ however pass an array to the writer method generated by `accepts_nested_attributes_for` if you're using plain Ruby or some other API access. See (TODO) for more info and example.